When you log in to Facebook through the game your Hearts are automatically added to your balance and your Dragon Skin will also be available. 

Please check out our video which shows you how: https://youtu.be/7yP9gO0UlGc

To check or change your Dragon Skin click on the Upgrade button at the bottom left of the screen then on Blaze and then you will find a button at the bottom of the screen ‘Change Skins’. When you click on Change Skins you will see all the available Skins for Blaze and your Rainbow Skin will be there.